1. Field of the Invention
The present invention relates to an apparatus for encoding color image data obtained from a color image reader or a color television camera.
2. Related Background Art
In the transmission or storage of image information, it is customary to reduce the redundancy of said information by encoding, in order to improve the efficiency of said transmission or storage. Such encoding has mostly been applied to binary black-and-white or color information.
However, multi-level image information is being employed in recent years for reproducing finer images, and multi-level color information also has started to be used.
The encoding of such multi-level color image information has been conducted by applying the conventional encoding method for black-and-white information to each of three primary colors of red (R), green (G) and blue (B), or by quantization utilizing mutual correlation of three primary colors. The former method is naturally inefficient and may result in color aberration. The latter method is generally free from color aberration, but cannot provide a high efficiency because the correlation of three primary colors is too strong.
The present applicant therefore proposed, in U.S. Patent Application Ser. No. 066,119 filed on June 25, 1987, a process of encoding color image data containing color information and luminance information in each block of pixels.
Said encoding process for color image data separates the color image data into the luminance information and the color information, and encodes each said information into a code of a fixed length.
Human vision can distinguish an image of a high resolution from an image of continuous tone. When a color image having such mutually opposite properties is encoded with a fixed encoding process explained above, the compression efficiency of encoding is improved but the human visual characteristic is sacrificed if the length of code is selected to be too short. For example if the color information does not contain information on resolution, or particularly if the block contains an edge therein, there may result mixing of colors or failure in reproducing fine lines. On the other hand, a larger length of codes will provide an image matching closer to the human visual characteristic but reduces the efficiency of compression.